Integrated circuits often have devices that are operated under several operation voltages, and the devices can be categorized as low-voltage devices, medium-voltage devices, and high-voltage devices. The voltages corresponding to the low-voltage devices, the medium-voltage devices, and the high-voltage devices vary. For example, the low-voltage devices may be operated at 4.5V or below, the medium-voltage devices may be operated between about 4.5V and 18V, and the high-voltage devices may be operated at 18V or above.
To meet the needs of different devices, multiple power supply lines are formed in the integrated circuits to carry different power supply voltages, wherein the power supply voltages may be referred to as a full VDD (FVDD, which is a high voltage), a half VDD (which is also referred to as a medium voltage, or MV, since it is lower than the FVDD), and a low VDD (LVDD). In a high-voltage application, the power-on sequence is that power supply voltage FVDD is activated first. Power supply voltage HVDD, however, is activated at a slower pace than power supply voltage FVDD is. Accordingly, a weak time occurs when power supply voltage FVDD has aroused significantly, while power supply voltage HVDD has not. The voltage different between power supply voltages FVDD and the HVDD during the weak time may be much higher than the normal voltage difference (FVDD′−HVDD) during a normal operation time of the respective circuit. For example, during the normal operation of the integrated circuit, power supply voltage FVDD is 27V, and power supply voltage HVDD is 13.5V. The normal voltage difference (FVDD′−HVDD) is thus 13.5V. However, during the weak time, if power supply voltage FVDD has reached 27V, while power supply voltage HVDD reaches only 2V, the voltage difference is 25V. In the worst case scenario, when power supply voltage FVDD reaches it peak voltage, power supply voltage HVDD is still at 0V. The difference between power supply voltages FVDD and HVDD is thus 27V.
The weak time may be as long as 1 second. During which time, the medium-voltage devices that are coupled between power supply lines carrying power supply voltages FVDD and HVDD also suffer from the high voltage difference. Accordingly, the medium-voltage devices are over-stressed. This may cause reliability problems since the over-stressed medium-voltage devices may breakdown.
To improve the reliability of the medium-voltage devices, the medium-voltage devices may be designed to be able to sustain power supply voltage FVDD. This solution, however, causes the chip area occupied by the medium-voltage devices to increase significantly. In some cases, a 50 percent increase in the chip area may be needed. Another conventional method to improve the reliability is to forwardly couple a diode between a control bias voltage and the power supply line that carries power supply voltage HVDD. During the weak time, the control bias voltage is applied to the forward diode to pull up the HVDD rapidly, so that the difference between power supply voltages FVDD and HVDD is reduced during the weak time. Since the forwardly coupled diode may cause a high current, a resistor is further coupled between the control bias voltage and the power supply line that carries power supply voltage HVDD in order to limit the forward current. However, the resistance value of the resistor needs to be high, and hence the resistor occupies a large chip area.